1. Field of the Invention
The invention relates to a radio access scheme which considerably reduces the effect of multiple access interference in a code division multiple access (CDMA)-based communication system.
2. Description of the Related Art
A conventional code division multiple access (CDMA)-based communication system, for example a universal mobile communication system (UMTS), is interference-limited due to non-orthogonality of the spreading codes used for separating different users (or channels) of the same frequency band. In the uplink, users are further separated from each other by allocating a separate long scrambling code to each user. The codes are not orthogonal at a receiver due to the properties of the codes. In the downlink, users are separated by orthogonal Walsh/Hadamard spreading codes which remain orthogonal in a frequency-nonselective radio channel. On the other hand, in a frequency-selective radio channel the orthogonality is lost. The interference-limited nature of CDMA-based communication systems results in degraded receiver performance as the number of users in a cell increases. The degradation in receiver performance is caused by multiple access interference (MAI). MAI is present in both data signals and pilot signals. MAI in pilot signals results in decreased accuracy in channel estimation and synchronisation, for example.
MAI is caused by the non-orthogonality of the spreading codes in the receiver. Increased correlation between the spreading codes may be caused by a delay spread of a radio channel along with the properties of the spreading codes. Sub-optimal multi-user receivers have been developed to reduce the effect of MAI. Multi-user receivers may be classified into two main categories: linear equalizers and subtractive interference cancellation (IC) receivers. Linear equalizers, such as decorrelating detectors (i.e. zero-forcing equalizers) and minimum mean square error (MMSE) detectors, attempt to remove correlation between the spreading codes. The principle of an IC receiver is that a multiple access interference component is estimated and then subtracted from the received signal for making decisions more reliable.
Linear equalizers are not very suitable particularly in the up-link, when the number of simultaneously accessing users is high. IC receivers are usually complex, because the interference component is estimated by estimating the contribution of interference of each user and summing the contributions. Additionally, interference cancellation algorithms perform non-ideally mainly because of unreliable tentative bit decisions in estimation of interference contributed by a given a user, and because of errors in the channel estimation. Errors in channel estimation are caused by MAI appearing in pilot symbols, for example.